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Influence of Ta doping on the growth and superconducting properties of YBa2Cu3O6 + δ crystals
NH JO~O,CRYSTAL
GROWTH Journal of Crystal Growth 139 (1994) 190—195
ELSEVIER
Priority communication
Influence of Ta doping on the growth and superconducting properties of YBa2Cu3O6~~ crystals S.M. Rao
K.M. Chen
~
a,
K.S. Law a, S.R. Sheen CP. Khattak
a,
Y.F. Chen
h
M.K. Wu
Materials Science Center, National Tsing Hua Unicersity, Hsinchu 30043, Taiwan, ROC h , Ta liwa Junior College of Technology, Cliung Lw, Usinchu, Taiwan, RO( Crystal Systems, Salem, Massachusetts 01970, USA (Received 18 November 1993; manuscript received in final form
7 January 1994)
Abstract Single crystals of YBa~Cu3O~~, completely separated from flux, measuring up to 5—6 mm and 0.25—0.5 mm thick, were grown reproducibly from BaCuO, solutions containing 0.8 wt% Ta206, by the top seeded solution growth technique. Addition of Ta increases the crystal size due to reduced nucleation. The crystals exhibit a broad superconducting transition at 80 K when annealed in air (a narrow transition at 86 K on oxygen annealing). attributed to the incorporation of stoichiometric excess of oxygen in the presence of Ta, as supported by the EDX, thermogravimetry and oxygen titration analysis.
1. Introduction Large single crystals of the newly discovered oxide superconductor YBa2Cu1O6+a (henceforth called 123 for brevity) [1] are required to understand the phenomenon of superconductivity in these materials as well as for their possible applications. Even though small single crystals have been grown from high temperature solutions of BaCuO-, and its analogs by a number of workers [2-4], growth of large single crystals appears to be impeded by the high viscosity of the solutions and a narrow metastable region (1—3°C)of the liquid phase [5—7].As a result, even small tem-
_______
*
Corresponding author,
perature fluctuations cause crystallization on the surface of the solution leading to the growth of small crystallites in clusters, which stick to the growing crystal on the seed holder and lead to multiple crystallization [5]. In solution crystal growth this is generally overcome by the addition of small amounts of dopants [8,91 or altering the pH in case of aqueous solutions. Such information is not available for 123 and a number of additives were tried to suppress this parasitic crystallization. Polycrystalline samples doped with Nb and Ta did not require post-growth oxygen annealing and exhibited a sharp superconducting transition on simply annealing in air at 900°Cfor 10 to 12 h and cooling to room temperature [10,11]. We therefore studied the effect of adding Ta to the starting mixture of 123 and BaCuO-, on the morphology and superconducting properties
0022-0248/94/$07.00 © 1994 Elsevier Science By. All rights reserved SSD! 0022-0248(94)00042-K
SM. Rao et al. /Journal of Crystal Growth 139 (1994) 190—1 95
of the resulting crystals, and the results are presented here.
2. Experimental procedure 2.1. Crystal growth Crystals were grown by the top-seeded solution growth (TSSG) technique using an apparatus described elsewhere [12]. A 1: 5 mixture of 123 and BaCuO2, to which a predetermined amount of Ta205 (in the range 0.4 to 5 wt% of the total weight of the solution) was added, was filled into a high density recrystallized alumina crucible placed on a pedestal inside a vertical tubular furnace. The pedestal was rotated by a reciprocating motor unit whose rotation speed, duration of rotation in clockwise (CW) and counterclockwise (CCW) directions as well as hold time between reversals could be varied accurately [13]. In addition, a plate heater was used at the bottom of the crucible to dissolve the parasitic crystallization. This helps in maintaining the homogeneity of the solution. However, the crucible rotation led to the dissolution of alumina in the solution and its incorporation into the growing crystal, besides covering the ingots width hexagonal crystals of BaA12O4. This problem was overcome by using high-density alumina crucibles of high surface quality, prepared in the laboratory. Presence of a small amount of Al did not affect other superconducting properties of the crystals grown
191
[2,3], except bringing the transition down by a few degrees. After placing the crucible containing the charge, the furnace temperature was slowly raised and held for 16 h at 1020°Cto homogenize the solution. It was then lowered to 990°C and allowed to stabilize for 4 h before an oriented sapphire rod was dipped into the solution to nucleate a seed. The crystal growth was carried out while cooling the solution at a rate of 1°C/ day. (The smallest rate of cooling provided by the programmer was only 0.1°C/h. Therefore, the furnace was cooled at this rate for 10 h and held for 14 h at the temperature reached before starting the next cooling cycle. This may therefore be treated as an average rate of cooling over the entire period.). After 8—10 days, the growing crystal was pulled out of the solution and annealed at 900—930°C.During this anneal it was found that the flux collecting on the surface of the crystalline mass moved to the cooler region on the seed holder and solidified in the shape of a disc, leaving the crystals free of flux, as shown in Fig. 1. Often the solution crept up to 8—10 cm along the alumina rod during crystal growth. SEM analysis showed 123 along with the flux at these heights. 2.2. Crystal characterization The growth and morphology of the crystals was examined using a JEOL JSM-600 scanning electron microscope (SEM) in conjunction with an
Fig. 1. As-grown ingots of the 123 crystals grown on sapphire rods. Flux sticking to the ingot moves up along the seed rod during the post-growth annealing at 900—930°Cand collects in the form of a disk above (below in figure) the cluster of crystals, leaving the surface of the crystals free of flux.
192
SM. Rao et a!. /Journal of Crystal Growth 139 (1994) /90 195
Oxford model Link eXL energy dispersive X-ray (EDX) analyzer. Resistivity of the single crystal samples extracted from the crystal cluster was
______
measured by the standard four-probe method. The crystals with silver contacts were initially annealed in air at 350 C for 12 h (for proper
__
__
bjl
_wI____
e)
Fig. 2. Change in the crystal growth of 123 as the concentration of Ta is varied: (a) undoped, (b) 0.4. (c) 0.8, (d) 1.5 and tel 3 wtVe of the charge. The size of the crystals increases up to 0.8~and also the clustering is reduced.
SM. Rao et al. /Journal of Crystal Growth 139 (1994) 1 90—1 95
adhesion), followed by annealing in oxygen at 500°Cfor 2—4 days. Oxygen stoichiometry in the powders annealing) sured by (without iodometricoxygen titration techniquewas [14]meaand the oxygen loss determined by thermogravimetric analysis.
193
60.00
0.25
0.20
~ ~
40 00
c~) c~
3. Results and discussion
.~
20.00
C.,)
(A)
3.1. Influence of Ta on the crystal growth Initial experiments showed that the crystalliza tion temperature increased from 985°C for an undoped solution to 993°Cfor a solution containing 5 wt% Ta2O5, indicating a possible increase in the solubility of 123 in BaCuO2 [8,9], probably because of the formation of complexes such as Ba(Y1~Ta~)O3 observed in the powder samples [15,16]. At high concentrations of Ta, large amounts of BaA12O4 crystals were observed, probably as a result of an interaction between the Ta and the alumina crucible. Similar crystalline phases containing Ta were not observed either among the crystal clusters or the solidified flux, as they might still be in solution with BaCuO2. The amount of Ta and Al present in the crystals as obtained from the EDX analysis is shown in Table 1. Increase in the Ta concentration in the crystals appears to be marginal compared to the actual amounts added to the solution, probably due a limit of solubility of Ta in the crystals. Portions of the as-grown crystal from each ingot, shown in the SEM micrographs of Figs. 2a—2e, consist of a cluster of crystals, but are well separated from the flux. It has not been possible Table 1 Ta concentration obtained from the EDX measurements on 123 crystals doped with different amounts of Ta205 the concentration of Al also given; all quantities are in wt% Sample No.
Ta205 added to solution
Ta concentration in crystal from EDX
Al concentration in crystal from EDX
TaO-4 0.4 0.16(1) 0.55(2) TaO-8 0.8 0.18(3) 1.11(2) TaO-15 1.5 0.19(5) 0.88(4) TaO-30 3.0 0.21(2) 1.06(2) ___________________________________________
____j0.00
0.05
-~
-__________
0.00
0
~~oo
100
300
Temperature (K)
I~Q~5
0.10
.—..
0.08
E .2-
0.06
~ ci ~
0.04
0.030
~
0.020 a)
0.015 b)
.~ CI~
0.02
0 010 0.005
(B)
0.00 0
100
200
300
Temperature (K) Fig. 3. The resistivity plots of (A) undoped and (B) Ta-doped 123 crystals: (a) and (b) represent curves obtained without and with post-growth oxygen annealing, respectively.
to obtain an ingot containing only one crystal due to the nucleation [5,6]. All the crystals were grown under identical conditions over the same duration and therefore, the size of the crystals as seen in the photographs under the same magnification can be taken to represent the approximate growth rate. As seen from Fig. 2, the crystals grown without Ta are the smallest and are most clustered due to the granulation effects [5]. As the Ta content in the solution was increased, the crystals increased in size up to 0.8 wt% Ta. Addition of
194
SM. Rao et al. /Journal
of
Crystal (,rowth /39 (1994) 190—195
more Ta did not increase the crystal size, but led to excessive growth of hexagonal shaped BaAl2O4 crystals that deposited on the surface of the cluster. At 5 wt% Ta, the cluster was completely covered by a mass of flux. Therefore, the optimum concentration of Ta206 is around 0.8% by weight of the solution to obtain good crystals.
0.010
0.-i -
0.0
- -
‘l’a~()5
svtZ
~
wt.%
-~ ... -
~
I
,,
OIL))
~2 Influcnce of Ta on the superconductln/, proper 001))
Figs. 3A and 3B show typical resistivity plots of 123 crystals grown without and with the addition of Ta to the flux, respectively. The Ta-doped sample shows a semiconducting behavior in the normal state resistivity with a broad superconducting transition at 80 K (~i 40 K) even without oxygen annealing. On oxygen annealing for 48 h at 500 C, the transition is observed at 86 K with a reduced width. On the other hand, the undoped crystal shows superconductivity only on postgrowth oxygen annealing for 96 h. Much longer time is required for thick crystals [2,3]. Both the doped and undoped samples exhibit a metallic behavior in the normal state only after oxygen annealing. The resistivity plots of 123 crystals grown from solutions containing different amounts of Ta, and subjected to post-growth oxygen annealing for 48 h at 500°Care shown in Fig. 4. The onset, observed at 91 K in TaO-4, is found to decrease marginally as the Ta content increases to 86 K in the case of TaO-30. On the other hand, the width of the transition increases with increasing Ta content, from 4 K for TaO-4 to 10 K for TaO-30, probably due to some lower temperature phases formed in the presence of excess Ta or oxygen
may be •btd to the presence of ~23h measured by the EDX analysis.
-
0.000
~-‘-‘~-‘
-
-~-~---~
- -
200
1 00
:300
Telflperature (K)
Fig. 4. Resistivity plots of 123 crystals doped with dilterent amounts of Ta,06: (— — —) 0.4’ ( ) 08’ (— -——) I ~ md
) 3 wt% T~0
,
-
temperature (800°C) than the normal samples (400°C). An increase in weight observed in the 450—600°Crange is probably a result of oxygen pickup by the sample. Results of the oxygen titration analysis of the Ta doped 123 powders presented in Fig. 6 show that the samples contain a stoichiometric excess of oxygen. Powder X-ray diffraction analysis [16] of the samples shows that they correspond to YBa2Cu3O6,8, corroborating the above result. Thus, the crystals contain a stoichiometric excess
0.0 24)
as
-1.0 937
3.3. Oxygen stoichiometty Thermogravimetric analysis of the Ta-doped 123 powder sintered in air at 900°Cand furnacecooled to room temperature, shown in Fig. indicates that oxygen loss occurs at a much higher
~-
0
-2.4) ~44
I ll3(~
-
SoIl
—404) Jeilil)(r’iItiI’(’
.
-24)
11114)
I
.
Fig. ~. Thermogravimetric analysis a powder containing 0.4 wt% Ta~O~ showing thatofthe sample sample is stabilized
up to 801CC.
SM. Rao et al. /Journal of Crystal Growth 139 (1994) 1 90—1 95
5. Acknowledgments
70
We thank Ms. E.H. Liu for helping with the SEM and EDX analysis and Mr. R. C. Chen for help in setting up the crystal growing systems. One of the authors (S.M.R.) is thankful to the National Science Council of the ROC for financial support. This research was supported under the ROC National Science Council Grant #NSC82-05 1 1-M-007-140.
E 0 .~
195
6.9
0 5) >~
0 68 0
I
Tantalum
2
3
4
5
oxide concentration c (wt %)
Fig. 6. lodometric titration analysis of the variation of the oxygen content y and the concentration c of Ta 205 in different powder samples.
of oxygen in the presence of pentavalent Ta, but are still oxygen deficient as they do not show the metallic behavior in the normal state, while exhibiting a broad superconducting transition at 80 K (.~ 40 K). ‘~
6. References [1] M.K. Wu, JR. Asburn, C.J. Torng, PH. Horr, R.L. Meng, L. Gao, Z.J. Huang and C.W. Chu, Phys. Rev. Lett. 58, 908 (1987). [2] Hi. Scheel and F. Licci, J. Crystal Growth 85 (1987) 607. [31SM. Rao, B.H. Loo, NP. Wang and Ri. Kelley, J. Crystal Growth 110 (1991) 989. [4] R. Gagnon, M. Oussena and M. Aubin, J. Crystal Growth 121 (1992)
4. Conclusion Crystals exhibiting a plate-type morphology, and measuring 5—6 mm square and 0.25—0.5 mm thick, could be grown reproducibly from BaCuO2 solutions containing Ta2O5. Addition of Ta also helps in incorporating and stabilizing stoichiometric excess of oxygen in 123. As a result the crystals exhibit a broad superconducting transition at 80 K even without oxygen annealing. On oxygen annealing, however, the transition ternperature is increased and the width reduced. Thus Ta helps in improving the growth rate and morphology of the 123 crystals and also improves their superconducting properties. However, detailed investigation of the doped crystals, and with more dopants, is needed for an understanding of the influence of the dopants on the defects in the crystals.
559.
[51SM.
Rao, G.M. Chen and M.K. Wu, 9th American Crystal Growth Conf. (ACCG-9), Baltimore, MD, 1993. [6] SM. Rao, G.M. Chen and M.K. Wu, in: Proc. Chinese Soc. for Materials Science, Hsinchu, 1993, p. 7—47. [7] A.A. Zhokov and G.A. Emel’chenko, J. Crystal Growth 129 (1993) 786. [81D. Elwell, in: Crystal Growth, Ed. BR. Pamplin (Pergamon, Oxford, 1975) p. 185. [9] B.M. Wanklyn, in: Crystal Growth. Ed. BR. Pamplin (Pergamon, Oxford, 1975) p. 217. [10] Ky. Paulose, J. Koshay and AD. Damodaran, Jap. J. AppI. Phys. 30 (1991) L458. [11] S.M. Rao, M.J. Wang, SR. Sheen, M.K. Wu and Y.F. Chen, unpublished. [12] SM. Rao, M.P. Chougaonkar and M. Shukla, Indian J. Phys. 54B (1980) 257. [131SM. Rao, KM. Chen and M.K. Wu, unpublished. [14] E.H. Appelman, L.R. Morse, U. Geiser, A. Umezawa, G.W. Crabtree and K.D. Karlson, Inorg. Chem. 26 (1987)
3239,
[15] N. Brenicivic, Lj. Tusek-Bozic, S. Popovic, B. Rakavin, G. Leising, K.D. Aichholzer, E. Schweiger and V. Wippel, J. Less-Common Metals 166 (1990) 63. [16] Y. Ping, PH. Kun and SM. Ran, unpublished.
GROWTH Journal of Crystal Growth 139 (1994) 190—195
ELSEVIER
Priority communication
Influence of Ta doping on the growth and superconducting properties of YBa2Cu3O6~~ crystals S.M. Rao
K.M. Chen
~
a,
K.S. Law a, S.R. Sheen CP. Khattak
a,
Y.F. Chen
h
M.K. Wu
Materials Science Center, National Tsing Hua Unicersity, Hsinchu 30043, Taiwan, ROC h , Ta liwa Junior College of Technology, Cliung Lw, Usinchu, Taiwan, RO( Crystal Systems, Salem, Massachusetts 01970, USA (Received 18 November 1993; manuscript received in final form
7 January 1994)
Abstract Single crystals of YBa~Cu3O~~, completely separated from flux, measuring up to 5—6 mm and 0.25—0.5 mm thick, were grown reproducibly from BaCuO, solutions containing 0.8 wt% Ta206, by the top seeded solution growth technique. Addition of Ta increases the crystal size due to reduced nucleation. The crystals exhibit a broad superconducting transition at 80 K when annealed in air (a narrow transition at 86 K on oxygen annealing). attributed to the incorporation of stoichiometric excess of oxygen in the presence of Ta, as supported by the EDX, thermogravimetry and oxygen titration analysis.
1. Introduction Large single crystals of the newly discovered oxide superconductor YBa2Cu1O6+a (henceforth called 123 for brevity) [1] are required to understand the phenomenon of superconductivity in these materials as well as for their possible applications. Even though small single crystals have been grown from high temperature solutions of BaCuO-, and its analogs by a number of workers [2-4], growth of large single crystals appears to be impeded by the high viscosity of the solutions and a narrow metastable region (1—3°C)of the liquid phase [5—7].As a result, even small tem-
_______
*
Corresponding author,
perature fluctuations cause crystallization on the surface of the solution leading to the growth of small crystallites in clusters, which stick to the growing crystal on the seed holder and lead to multiple crystallization [5]. In solution crystal growth this is generally overcome by the addition of small amounts of dopants [8,91 or altering the pH in case of aqueous solutions. Such information is not available for 123 and a number of additives were tried to suppress this parasitic crystallization. Polycrystalline samples doped with Nb and Ta did not require post-growth oxygen annealing and exhibited a sharp superconducting transition on simply annealing in air at 900°Cfor 10 to 12 h and cooling to room temperature [10,11]. We therefore studied the effect of adding Ta to the starting mixture of 123 and BaCuO-, on the morphology and superconducting properties
0022-0248/94/$07.00 © 1994 Elsevier Science By. All rights reserved SSD! 0022-0248(94)00042-K
SM. Rao et al. /Journal of Crystal Growth 139 (1994) 190—1 95
of the resulting crystals, and the results are presented here.
2. Experimental procedure 2.1. Crystal growth Crystals were grown by the top-seeded solution growth (TSSG) technique using an apparatus described elsewhere [12]. A 1: 5 mixture of 123 and BaCuO2, to which a predetermined amount of Ta205 (in the range 0.4 to 5 wt% of the total weight of the solution) was added, was filled into a high density recrystallized alumina crucible placed on a pedestal inside a vertical tubular furnace. The pedestal was rotated by a reciprocating motor unit whose rotation speed, duration of rotation in clockwise (CW) and counterclockwise (CCW) directions as well as hold time between reversals could be varied accurately [13]. In addition, a plate heater was used at the bottom of the crucible to dissolve the parasitic crystallization. This helps in maintaining the homogeneity of the solution. However, the crucible rotation led to the dissolution of alumina in the solution and its incorporation into the growing crystal, besides covering the ingots width hexagonal crystals of BaA12O4. This problem was overcome by using high-density alumina crucibles of high surface quality, prepared in the laboratory. Presence of a small amount of Al did not affect other superconducting properties of the crystals grown
191
[2,3], except bringing the transition down by a few degrees. After placing the crucible containing the charge, the furnace temperature was slowly raised and held for 16 h at 1020°Cto homogenize the solution. It was then lowered to 990°C and allowed to stabilize for 4 h before an oriented sapphire rod was dipped into the solution to nucleate a seed. The crystal growth was carried out while cooling the solution at a rate of 1°C/ day. (The smallest rate of cooling provided by the programmer was only 0.1°C/h. Therefore, the furnace was cooled at this rate for 10 h and held for 14 h at the temperature reached before starting the next cooling cycle. This may therefore be treated as an average rate of cooling over the entire period.). After 8—10 days, the growing crystal was pulled out of the solution and annealed at 900—930°C.During this anneal it was found that the flux collecting on the surface of the crystalline mass moved to the cooler region on the seed holder and solidified in the shape of a disc, leaving the crystals free of flux, as shown in Fig. 1. Often the solution crept up to 8—10 cm along the alumina rod during crystal growth. SEM analysis showed 123 along with the flux at these heights. 2.2. Crystal characterization The growth and morphology of the crystals was examined using a JEOL JSM-600 scanning electron microscope (SEM) in conjunction with an
Fig. 1. As-grown ingots of the 123 crystals grown on sapphire rods. Flux sticking to the ingot moves up along the seed rod during the post-growth annealing at 900—930°Cand collects in the form of a disk above (below in figure) the cluster of crystals, leaving the surface of the crystals free of flux.
192
SM. Rao et a!. /Journal of Crystal Growth 139 (1994) /90 195
Oxford model Link eXL energy dispersive X-ray (EDX) analyzer. Resistivity of the single crystal samples extracted from the crystal cluster was
______
measured by the standard four-probe method. The crystals with silver contacts were initially annealed in air at 350 C for 12 h (for proper
__
__
bjl
_wI____
e)
Fig. 2. Change in the crystal growth of 123 as the concentration of Ta is varied: (a) undoped, (b) 0.4. (c) 0.8, (d) 1.5 and tel 3 wtVe of the charge. The size of the crystals increases up to 0.8~and also the clustering is reduced.
SM. Rao et al. /Journal of Crystal Growth 139 (1994) 1 90—1 95
adhesion), followed by annealing in oxygen at 500°Cfor 2—4 days. Oxygen stoichiometry in the powders annealing) sured by (without iodometricoxygen titration techniquewas [14]meaand the oxygen loss determined by thermogravimetric analysis.
193
60.00
0.25
0.20
~ ~
40 00
c~) c~
3. Results and discussion
.~
20.00
C.,)
(A)
3.1. Influence of Ta on the crystal growth Initial experiments showed that the crystalliza tion temperature increased from 985°C for an undoped solution to 993°Cfor a solution containing 5 wt% Ta2O5, indicating a possible increase in the solubility of 123 in BaCuO2 [8,9], probably because of the formation of complexes such as Ba(Y1~Ta~)O3 observed in the powder samples [15,16]. At high concentrations of Ta, large amounts of BaA12O4 crystals were observed, probably as a result of an interaction between the Ta and the alumina crucible. Similar crystalline phases containing Ta were not observed either among the crystal clusters or the solidified flux, as they might still be in solution with BaCuO2. The amount of Ta and Al present in the crystals as obtained from the EDX analysis is shown in Table 1. Increase in the Ta concentration in the crystals appears to be marginal compared to the actual amounts added to the solution, probably due a limit of solubility of Ta in the crystals. Portions of the as-grown crystal from each ingot, shown in the SEM micrographs of Figs. 2a—2e, consist of a cluster of crystals, but are well separated from the flux. It has not been possible Table 1 Ta concentration obtained from the EDX measurements on 123 crystals doped with different amounts of Ta205 the concentration of Al also given; all quantities are in wt% Sample No.
Ta205 added to solution
Ta concentration in crystal from EDX
Al concentration in crystal from EDX
TaO-4 0.4 0.16(1) 0.55(2) TaO-8 0.8 0.18(3) 1.11(2) TaO-15 1.5 0.19(5) 0.88(4) TaO-30 3.0 0.21(2) 1.06(2) ___________________________________________
____j0.00
0.05
-~
-__________
0.00
0
~~oo
100
300
Temperature (K)
I~Q~5
0.10
.—..
0.08
E .2-
0.06
~ ci ~
0.04
0.030
~
0.020 a)
0.015 b)
.~ CI~
0.02
0 010 0.005
(B)
0.00 0
100
200
300
Temperature (K) Fig. 3. The resistivity plots of (A) undoped and (B) Ta-doped 123 crystals: (a) and (b) represent curves obtained without and with post-growth oxygen annealing, respectively.
to obtain an ingot containing only one crystal due to the nucleation [5,6]. All the crystals were grown under identical conditions over the same duration and therefore, the size of the crystals as seen in the photographs under the same magnification can be taken to represent the approximate growth rate. As seen from Fig. 2, the crystals grown without Ta are the smallest and are most clustered due to the granulation effects [5]. As the Ta content in the solution was increased, the crystals increased in size up to 0.8 wt% Ta. Addition of
194
SM. Rao et al. /Journal
of
Crystal (,rowth /39 (1994) 190—195
more Ta did not increase the crystal size, but led to excessive growth of hexagonal shaped BaAl2O4 crystals that deposited on the surface of the cluster. At 5 wt% Ta, the cluster was completely covered by a mass of flux. Therefore, the optimum concentration of Ta206 is around 0.8% by weight of the solution to obtain good crystals.
0.010
0.-i -
0.0
- -
‘l’a~()5
svtZ
~
wt.%
-~ ... -
~
I
,,
OIL))
~2 Influcnce of Ta on the superconductln/, proper 001))
Figs. 3A and 3B show typical resistivity plots of 123 crystals grown without and with the addition of Ta to the flux, respectively. The Ta-doped sample shows a semiconducting behavior in the normal state resistivity with a broad superconducting transition at 80 K (~i 40 K) even without oxygen annealing. On oxygen annealing for 48 h at 500 C, the transition is observed at 86 K with a reduced width. On the other hand, the undoped crystal shows superconductivity only on postgrowth oxygen annealing for 96 h. Much longer time is required for thick crystals [2,3]. Both the doped and undoped samples exhibit a metallic behavior in the normal state only after oxygen annealing. The resistivity plots of 123 crystals grown from solutions containing different amounts of Ta, and subjected to post-growth oxygen annealing for 48 h at 500°Care shown in Fig. 4. The onset, observed at 91 K in TaO-4, is found to decrease marginally as the Ta content increases to 86 K in the case of TaO-30. On the other hand, the width of the transition increases with increasing Ta content, from 4 K for TaO-4 to 10 K for TaO-30, probably due to some lower temperature phases formed in the presence of excess Ta or oxygen
may be •btd to the presence of ~23h measured by the EDX analysis.
-
0.000
~-‘-‘~-‘
-
-~-~---~
- -
200
1 00
:300
Telflperature (K)
Fig. 4. Resistivity plots of 123 crystals doped with dilterent amounts of Ta,06: (— — —) 0.4’ ( ) 08’ (— -——) I ~ md
) 3 wt% T~0
,
-
temperature (800°C) than the normal samples (400°C). An increase in weight observed in the 450—600°Crange is probably a result of oxygen pickup by the sample. Results of the oxygen titration analysis of the Ta doped 123 powders presented in Fig. 6 show that the samples contain a stoichiometric excess of oxygen. Powder X-ray diffraction analysis [16] of the samples shows that they correspond to YBa2Cu3O6,8, corroborating the above result. Thus, the crystals contain a stoichiometric excess
0.0 24)
as
-1.0 937
3.3. Oxygen stoichiometty Thermogravimetric analysis of the Ta-doped 123 powder sintered in air at 900°Cand furnacecooled to room temperature, shown in Fig. indicates that oxygen loss occurs at a much higher
~-
0
-2.4) ~44
I ll3(~
-
SoIl
—404) Jeilil)(r’iItiI’(’
.
-24)
11114)
I
.
Fig. ~. Thermogravimetric analysis a powder containing 0.4 wt% Ta~O~ showing thatofthe sample sample is stabilized
up to 801CC.
SM. Rao et al. /Journal of Crystal Growth 139 (1994) 1 90—1 95
5. Acknowledgments
70
We thank Ms. E.H. Liu for helping with the SEM and EDX analysis and Mr. R. C. Chen for help in setting up the crystal growing systems. One of the authors (S.M.R.) is thankful to the National Science Council of the ROC for financial support. This research was supported under the ROC National Science Council Grant #NSC82-05 1 1-M-007-140.
E 0 .~
195
6.9
0 5) >~
0 68 0
I
Tantalum
2
3
4
5
oxide concentration c (wt %)
Fig. 6. lodometric titration analysis of the variation of the oxygen content y and the concentration c of Ta 205 in different powder samples.
of oxygen in the presence of pentavalent Ta, but are still oxygen deficient as they do not show the metallic behavior in the normal state, while exhibiting a broad superconducting transition at 80 K (.~ 40 K). ‘~
6. References [1] M.K. Wu, JR. Asburn, C.J. Torng, PH. Horr, R.L. Meng, L. Gao, Z.J. Huang and C.W. Chu, Phys. Rev. Lett. 58, 908 (1987). [2] Hi. Scheel and F. Licci, J. Crystal Growth 85 (1987) 607. [31SM. Rao, B.H. Loo, NP. Wang and Ri. Kelley, J. Crystal Growth 110 (1991) 989. [4] R. Gagnon, M. Oussena and M. Aubin, J. Crystal Growth 121 (1992)
4. Conclusion Crystals exhibiting a plate-type morphology, and measuring 5—6 mm square and 0.25—0.5 mm thick, could be grown reproducibly from BaCuO2 solutions containing Ta2O5. Addition of Ta also helps in incorporating and stabilizing stoichiometric excess of oxygen in 123. As a result the crystals exhibit a broad superconducting transition at 80 K even without oxygen annealing. On oxygen annealing, however, the transition ternperature is increased and the width reduced. Thus Ta helps in improving the growth rate and morphology of the 123 crystals and also improves their superconducting properties. However, detailed investigation of the doped crystals, and with more dopants, is needed for an understanding of the influence of the dopants on the defects in the crystals.
559.
[51SM.
Rao, G.M. Chen and M.K. Wu, 9th American Crystal Growth Conf. (ACCG-9), Baltimore, MD, 1993. [6] SM. Rao, G.M. Chen and M.K. Wu, in: Proc. Chinese Soc. for Materials Science, Hsinchu, 1993, p. 7—47. [7] A.A. Zhokov and G.A. Emel’chenko, J. Crystal Growth 129 (1993) 786. [81D. Elwell, in: Crystal Growth, Ed. BR. Pamplin (Pergamon, Oxford, 1975) p. 185. [9] B.M. Wanklyn, in: Crystal Growth. Ed. BR. Pamplin (Pergamon, Oxford, 1975) p. 217. [10] Ky. Paulose, J. Koshay and AD. Damodaran, Jap. J. AppI. Phys. 30 (1991) L458. [11] S.M. Rao, M.J. Wang, SR. Sheen, M.K. Wu and Y.F. Chen, unpublished. [12] SM. Rao, M.P. Chougaonkar and M. Shukla, Indian J. Phys. 54B (1980) 257. [131SM. Rao, KM. Chen and M.K. Wu, unpublished. [14] E.H. Appelman, L.R. Morse, U. Geiser, A. Umezawa, G.W. Crabtree and K.D. Karlson, Inorg. Chem. 26 (1987)
3239,
[15] N. Brenicivic, Lj. Tusek-Bozic, S. Popovic, B. Rakavin, G. Leising, K.D. Aichholzer, E. Schweiger and V. Wippel, J. Less-Common Metals 166 (1990) 63. [16] Y. Ping, PH. Kun and SM. Ran, unpublished.